Australian researchers are warning of a new, invasive threat to the continent’s native wildlife: goldfish that were abandoned by their owners and released into the wild. Most of us think of goldfish as a small and harmless species, but apparently Western Australia’s rivers contain just the right conditions to allow the fish to grow into two kilo monsters that wreak havoc on the local ecosystem.

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There are a number of reasons why these fish pose such an environmental hazard. For one, they tend to eat the eggs of native species. But even when they aren’t directly affecting the reproduction of other fish, they’re releasing a nutrient-rich waste into the water column which creates dangerous algae blooms. They’re also carriers of nasty diseases that don’t naturally occur in Australia’s waters.

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It’s believed that pet owners who dump unwanted fish in local waterways are to blame. The practice is called “aquarium dumping.” Once they are released into the water, they breed at a rapid rate, taking over the area. Because they can travel quite far, up to 230 kilometers per year, they’re incredibly difficult to eradicate. In fact, scientists from Murdoch University are calling them “one of the world’s worst invasive aquatic species.”

This isn’t the first time pet goldfish have caused an ecological crisis. In 2013, researchers at Lake Tahoe in the US found abandoned goldfish that had grown over and foot and a half long terrorizing the waters.

Via Gizmodo

Images via Murdoch University